Fiber amplifiers are commonly used in many applications, including telecommunications applications and high power military and industrial fiber optic applications. For example, both U.S. Pat. No. 5,946,130, issued Aug. 31, 1999 to Rice and U.S. Pat. No. 5,694,408 issued Dec. 2, 1997 to Bott et al. describe many such applications in which fiber amplifiers are employed including the processing of materials, laser weapon and laser ranging systems, and a variety of medical and other applications.
Optical fiber amplifiers are designed to increase the power output levels of the signals propagating therealong. One conventional optical fiber amplifier design is an end-pumped dual-clad fiber, such as that described in U.S. Pat. No. 4,815,079 issued Mar. 21, 1989 to Snitzer et al. Referring to FIGS. 1A and 1B, the dual-clad fiber 10 has a single mode signal core 12, a multi-mode pump core 14 surrounding the signal core, and an outer cladding layer 16 surrounding the pump core for confining pump energy within the pump core, such that signals propagating through the signal core are amplified. The signal core will typically be doped with one or more rare earth elements such as, for example, ytterbium, neodymium, praseodymium, erbium, holmium or thulium.
In operation, pump energy is coupled into the pump core 14 at the input end 18 of the fiber. The pump energy then propagates along through the pump core until it is absorbed by the dopant in the signal core 12, thus amplifying signals propagating through the signal core. Although dual-clad fibers 10 can have different sizes, one typical dual-clad fiber includes a signal core that has a diameter of 8-10 μm and a pump core that has cross-sectional dimensions of 100-300 μm. End-pumped dual-clad fiber amplifiers of this size can typically reach fiber energy power levels of 115 W.
Due to the nature of optical radiation, the pump energy (shown as E) has different characteristics in two characteristic directions. As the pump energy propagates at least a portion of the pump energy diverges in a fast direction 17, and at least a portion of the pump energy diverges in a slow direction 19, with the pump energy diverging in each direction at a different rate. As such, the pump energy is typically coupled into the pump core using a fast axis lens that collimates the pump energy diverging in the direction established by the fast axis. But because the pump energy diverges in each direction at different rates, the fast axis lens generally does not adequately focus the pump energy diverging in the slow direction. Conventionally, no effort is made to focus the pump energy diverging in the slow direction. And as such, the net effect produces a distribution of pump energy at the input end of the fiber that unequally diverges in each direction which, in turn, reduces the efficiency of pump energy coupled into the pump core.
Considering the unequal divergence of pump energy as the pump energy is coupled into the pump core, a number of different approaches have been taken to otherwise allow the largest amount of pump energy as possible to be coupled into the end of the fiber. For example, one approach that has been taken is to design a fiber having as large a pump core as practical. But because of the requirement to maintain a significant absorption of pump energy per unit length of fiber, the size of the pump core is limited.
In addition to increasing the size of the pump core to couple the largest amount of pump energy into the end of the fiber, a number of alternative pumping techniques have been developed. For example, U.S. Pat. No. 5,854,865 issued Dec. 29, 1998 to Goldberg discloses a fiber amplifier having a v-shaped notch cut into the pump core through the cladding layer. Pump energy can then be reflected or refracted from one of the angled faces of the v-shaped notch so as to be injected directly into the pump core. Another technique involves the use of a fiber amplifier having portions of the cladding and the pump core removed. The fiber amplifier is then spooled between two reflective elements and pump energy introduced into the region between the reflective elements. The pump energy is then repeatedly reflected by the reflective elements in order to amplify signals propagating through the signal core.
While the current techniques have achieved some level of success, they still do not address the issue of the unequal diverging of pump energy into the fiber. And as such, the current techniques do not most efficiently couple pump energy into the pump core. Therefore, it would be desirable to design a fiber amplifier that accommodated the pump energy diverging in the slow direction, as well as the pump energy diverging in the fast direction, to thereby couple pump energy into the pump core more efficiently than current techniques.